Two-stage plasma process for converting waste into fuel gas and apparatus therefor

ABSTRACT

A two-step gasification process and apparatus for the conversion of solid or liquid organic waste into clean fuel, suitable for use in a gas engine or a gas burner, is described. The waste is fed initially into a primary gasifier, which is a graphite arc furnace. Within the primary gasifier, the organic components of the waste are mixed with a predetermined amount of air, oxygen or steam, and converted into volatiles and soot. The volatiles consist mainly of carbon monoxide and hydrogen, and may include a variety of other hydrocarbons and some fly ash. The gas exiting the primary gasifier first passes through a hot cyclone, where some of the soot and most of the fly ash is collected and returned to the primary gasifier. The remaining soot along with the volatile organic compounds is further treated in a secondary gasifier where the soot and the volatile compounds mix with a high temperature plasma jet and a metered amount of air, oxygen or steam, and are converted into a synthesis gas consisting primarily of carbon monoxide and hydrogen. The synthesis gas is then quenched and cleaned to form a clean fuel gas suitable for use in a gas engine or a gas burner. This offers higher thermal efficiency than conventional technology and produces a cleaner fuel than other known alternatives.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and apparatus for a two-stageconversion of organic components contained in solid and/or liquid waste,at high plasma temperature, into a fuel gas suitable for use in a gasengine or turbine for the production of electricity or a gas burner forthe production of steam, or in chemical synthesis reactions.

2. Description of the Prior Art

Numerous methods have been proposed for the conversion of waste intoenergy. The most common method is incineration. In incineration systems,waste is typically introduced in a high temperature chamber and reactedwith large amounts of air. The process can be one stage or two stages.Whether the incineration process is one stage or two stages, the processalways uses large amounts of air, resulting in the production of largeamounts of hot off-gas, typically laden with entrained particulates andacid gas components. Thermal energy is typically extracted from this hotdust-laden acid gas using a heat recovery boiler.

This method of extracting energy from a hot dirty gas is subject to twomain problems. First, heat recovery boilers are subject to corrosionfrom the acid gas and fouling from the particulates, especially abovetemperatures of 700° C. Second, the slow cooling of gas in a recoveryboiler is the major cause for the de novo synthesis of dioxins thatoccurs in the temperature range of 250-400° C. (c.f Cernuschi et al.,“PCDDIF and Trace Metals Balance in a MSW Incineration Full ScalePlant”, Proceeding of the 2000 International Conference on Incinerationand Thermal Treatment Technologies). Thus, energy cannot be safelyrecovered at temperatures below 400° C. because of the risk of formingdioxins. In a typical incinerator, gases exit the main incinerationchamber at 1100° C. and exit the chimney at 150° C. Of this range,energy can only be practically and safely recovered in the range from700 to 400° C., meaning that only about one third of the availableenergy can be recovered. Solutions have been proposed to alleviate someof these problems in incineration. For example, U.S. Pat. No. 5,092,254of Kubin et al. proposes a process whereby lime is injected in theincinerator to neutralize the acid gases and reduce corrosion levelinside the incineratopand auxiliary equipment. U.S. Pat. No. 5,797,336of Müller et al. discloses a process for the incineration of wastematerial whereby waste is first incinerated in a furnace chamber, thenre-burnt in a fluidized bed afterburning chamber, in order to reduce thenumber of particulates, and go to a heat recovery boiler where the gastemperature is reduced from 700-1100° C. to 100-300° C. Fundamentally,however, all incineration systems try to extract energy from a hot dirtygas.

Also, by using an independent source of heating, such as plasma, a widerange of waste types can be combusted, independently of theircomposition. The plasma also allows reaching high temperatures that willmelt the inorganic components of the waste into an inert slag and willdissociate them from the organic components of the waste, which willform a gas.

A number of methods and apparatus have been proposed for thedecomposition of wastes, hazardous or not, into inert slag andnon-hazardous gases with the use of plasma. Thus, U.S. Pat. No.4,960,380 of Cheetham describes a two-step process, wherein in the firststep plasma is used to reduce solid waste materials to a slag-likematerial from which more harmful constituents have been removed and to agaseous effluvium. The effluvium of the plasma reduction process isscrubbed to remove particulates. The gas is then processed by additionalheating and oxygen addition in order to convert the carbon monoxide inthe gas into carbon dioxide. Products of incomplete combustion (and/orchemically harmful constituents) are also eliminated in this step. Theoxidized gas is then suitable for safely exhausting into the atmosphere.In this system, coherent radiation (laser) is used to generate andsustain the plasma. This process is targeted at treating low organiccontent waste, such as incinerator ash. Moreover, the gas exhausted fromthe process being a hot combustion gas, the problems associated withincineration, described above, also apply to this process.

A plasma torch can also be used as an independent source of heat. Forexample, U.S. Pat. No. 5,534,659 of Springer et al. describes a singlestep method and an apparatus for treating hazardous and non-hazardouswaste materials composed of organic and inorganic components bysubjecting them to high temperature pyrolysis and controlledgasification of organic materials and metals recovery and/orvitrification of inorganic materials. The source of heating for thereactor is a conventional plasma arc torch.

U.S. Pat. No. 5,451,738 of Alvi et al. provides a two-step method forthe disposal of waste material, including volatile components andvitrifiable components, by first heating the waste to vaporize thehydrocarbon liquids and thereafter feeding to a primary plasma reactoron the surface of a molten pool where the vitrifiable components aremelted and the volatile components are volatilized. The reactor isequipped with multiple AC plasma torches. The torches use copperelectrodes, which are water-cooled. The hydrocarbon liquids and thevolatilized components are then fed to a secondary plasma reactor wherethey are dissociated into their elemental components.

The use of a plasma torch in order to obtain high reaction temperaturesin the gas phase poses some problems. Plasma torches have relatively lowenergy efficiency, whereby 30 to 40% of the electric energy to the torchis typically lost to cool the electrodes. Moreover, the water-cooledtorch presents the risk of water leaks onto the molten slag inside thereactor, creating an explosion. By contrast, considerable improvement isproduced by using graphite rods to generate the plasma in an arcfurnace, since graphite can withstand extremely high temperatures(several thousands of degrees), no water cooling is required and theenergy efficiency of the graphite rod is nearly 100%. Also, the risk ofwater leaking into the furnace is eliminated because the graphite rodsneed no cooling.

For example, U.S. Pat. No. 4,431,612 by Bell et al. describes a singlestep method and an apparatus for treatment of solid, liquid and gaseousPCB's as well as other hazardous materials by introducing them into achamber and into contact with a molten bath maintained in such chamberby a DC electric arc, which maintains the temperature in excess of 1600°C. The obtained molten bath serves to promote the initial decompositionor volatilization of PCB's and other hazardous materials, resulting in agaseous product that comprises CO, CO₂, H₂, CH₄ and HCl.

However, Bell et al. do not try to produce fuel gas from waste. Instead,their objective is to dissociate the waste into simple molecules. Thisprocess of dissociation does not use oxygen addition and is done in onestep. Hence, this process and similar processes will lead to theproduction of large amounts of carbon soot.

The production of soot under these reducing conditions is well known aswas shown in U.S. Pat. No. 5,451,738 by Alvi et al. that identified thisproblem and tried to alleviate it by catching the carbon black (soot) ina cyclonic scrubber. Similarly, in U.S. Pat. No. 5,534,659 of Springeret al. the problem of soot formation is recognized and oxidant injectionis used to convert the soot to carbon monoxide.

SUMMARY OF THE INVENTION

In order to recover energy from waste in a clean and efficient way, atechnology different than incineration is proposed. In a gasificationsystem using plasma, waste is converted to a fuel gas consisting mainlyof carbon monoxide and hydrogen, by heating up the waste in anoxygen-starved atmosphere. The gas produced is then cleaned ofcontaminants such as soot, before it can be used as fuel to produceelectricity or steam.

In a gasification system, most of the energy from the waste is stored inthe form of chemical energy instead of sensible (or thermal) energy asis the case in an incineration system. The amount of gas produced by agasification system is typically four to five times less than the gasproduced in an incineration system. This gives the possibility ofquenching the gas from the gasification temperatures (800 to 1100° C.depending on system) down to below saturation using water quenching.This approach eliminates the problem of dioxin formation, which occursin the 250 to 400° C. range.

The objective of the present invention is to convert essentially all thewaste to fuel gas. For this purpose, in addition to a primary gasifier,where initial conversion of waste into fuel gas takes place, there is aneed for a second stage gasifier to convert the carbon soot in the gasto gaseous carbon monoxide; this second stage includes the addition ofmetered amounts of oxygen into the gasifier.

The energy efficiency is higher when air is added to gasify the waste,namely by reacting the waste with oxygen, rather than simplydissociating the waste into simple molecules. The chemical energy of theproducts of dissociation is typically much higher than the chemicalenergy of the waste being treated. This means that significant amountsof electrical energy must be used for dissociation. In the presentinvention, by adding carefully metered amounts of oxygen or air and/orsteam to the process, it is possible to limit the amount of electrical(or plasma) energy required for dissociation. In fact, the amount ofoxygen fed can be increased so that partial combustion of the wasteoccurs and the plasma requirements are much reduced. Electrical energyis an expensive form of energy and it is important to use it asefficiently as possible.

By contrast to known plasma waste treatment systems, in the presentinvention the plasma energy serves mainly two purposes: 1) to vitrify(or melt) the inorganic portion of waste in the primary gasifier whilepartially gasifying the organic components, and 2) to provide theactivation energy to complete the gasification reactions in thesecondary gasifier.

In essence, therefore, the present invention provides a two-stage plasmaprocess for converting waste having organic and inorganic componentsinto fuel gas, which comprises:

-   -   (a) in the first stage, vitrifying or melting the inorganic        components of the waste and partially gasifying the organic        components; and    -   (b) in the second stage, completing the gasification of the        organic components so as to convert them into fuel gas.

Moreover, a dust separation and removal step is normally providedbetween the two stages of the process.

Furthermore, the fuel gas produced in the second stage is usuallyquenched and cleaned to make it suitable for use in a gas engine orturbine for production of electricity or In a gas burner for productionof steam or In chemical synthesis reactions.

Generally, the first stage is carried out in a plasma arc furnace, andthe second stage is carried out in a secondary gasifier using a plasmatorch with addition of metered amounts of oxygen. The plasma arc furnaceis preferably a refractory lined, enclosed furnace provided with atleast one direct current graphite electrode adapted to generate a plasmaarc to a bath of liquid inorganic material originating from the wasteitself and located at the bottom of the furnace. This liquid inorganicmaterial comprises a slag layer which is maintained at a temperature ofat least 1500° C., usually a temperature between 1500° C. and 1650° C.,and a metal layer also maintained at such temperature of at least 1500°C. and is located under the slag layer.

The waste is introduced into the furnace on top of the liquid inorganicmaterial and the organic component in the waste reacts with air, oxygenand/or steam supplied to the furnace in a predetermined amount adaptedto achieve gasification of the organics in the waste into a primarysynthesis gas containing CO, H₂, CO₂ and N₂ if the waste containsnitrogen or if air is added to the furnace, and also containing somesoot, fly ash and complex organic molecules. The organic material in thewaste is preferably reacted in the furnace so as to form a layer ofpartially treated waste on top of the slag layer and fresh waste isintroduced into the furnace on top of said partially treated waste layerwhich is maintained at a temperature of between 700 and 800° C. andconstitutes a cold top for the fresh waste added to the furnace. Theprimary synthesis gas exiting from the furnace is subjected to dustseparation and removal in which dust particles larger than apredetermined size are separated and removed. These dust particles arethen normally recycled to the furnace, while the remainder of the gas isfed to the secondary gasifier.

The secondary gasifier is preferably equipped with a plasma torch firedeductor which ensures that the gas from the first stage of the processentering the secondary gasifier is exposed to a high temperature such asto transform essentially all soot present in the gas to CO and toconvert essentially all complex organic molecules to simpler moleculesCO, CO₂ and H₂. This high temperature to which the gas from the firststage is exposed in the secondary gasifier is usually between 900° C.and 1300° C., preferably around 1100° C., and it is achieved mainly bypartial oxidation of the gas from the first stage by injection ofpredetermined amounts of air, oxygen and/or steam to the eductor, whilethe plasma torch provides only a small fraction of the energy requiredfor maintaining said high temperature.

The fuel gas exiting the secondary gasifier is normally cooled down veryrapidly to a temperature below 100° C. so as to freeze the thermodynamicequilibrium of the gas and avoid production of secondary pollutants, andafter such cooling, the fuel gas may be subjected to a final cleaningoperation to remove any remaining contaminants.

The entire process is preferably carried out under a negative pressureto preclude exit of toxic fumes or of flammable materials from any unitoperations. Also, an oxygen starved environment is used in the processto preclude dioxin formation.

The present invention also provides for an apparatus for convertingwaste having organic and inorganic components into fuel gas, whichnormally includes:

-   -   (a) a primary gasifier comprising a refractory lined, enclosed        plasma arc furnace provided with at least one graphite        electrode; at least one inlet for feeding waste into the        furnace; means for feeding air, oxygen and/or a stem in metered        amounts into the furnace; and a gas take off port for primary        synthesis gas produced in said primary gasifier; said primary        gasifier being adapted to maintain layers of molten metal and        molten slag at the bottom of the furnace and on top of the        molten slag a layer of partially treated waste over which fresh        waste is fed; and said at least one graphite electrode is        positioned so as to generate a plasma are to the molten slag        present in the furnace during the operation; and    -   (b) a secondary gasifier to which the primary synthesis gas is        fed, said secondary gasifier being equipped with a plasma-torch        fired eductor which ensures that the primary synthesis gas        entering from the primary gasifier is exposed to a high        temperature such as to transform any soot present in said        primary gas into CO and to convert any complex organic molecules        to simpler molecules CO, CO₂ and H₂; means for supplying metered        amounts of air, oxygen and/or steam into the eductor; said        eductor leading to an insulated chamber with a minimal heat        loss; and an outlet being provided in said chamber for the fuel        gas resulting from the operation.

Preferably, the primary gasifier has two graphite electrodes, one ofwhich creates an arc between one electrode and the slag during theoperation, and a second arc is created from the slag to the secondelectrode.

The eductor provided in the secondary gasifier is preferably made of ahigh heat metal alloy or is refractory lined or water cooled, and isequipped with a plasma torch at its inlet.

The apparatus may further comprise a dust separator, such as a hotcyclone, between the primary gasifier and the secondary gasifier, and agas quenching and gas cleaning means following the secondary gasifier.It may also be equipped with an induced draft fan adapted to operate theapparatus under a negative pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred, non-limitative embodiment of the invention will now bedescribed with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic representation of a preferred embodiment of thepresent invention;

FIG. 2 is an elevation section view of a preferred embodiment of theprimary gasifier used within the process and apparatus of the presentinvention; and

FIG. 3 is an elevation section view of a preferred embodiment of thesecondary gasifier used within the process and apparatus of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The process of the present invention can be used to process varioustypes of industrial, hazardous or domestic waste in the form of liquidsor solids. The solid wastes can be hospital waste, mixed plastics waste,municipal solid waste, automobile shredder residue or the like. Theliquid wastes can be spent solvents, used oils, petroleum sludge,municipal water treatment sludge, de-inking sludge or similar liquids.Normally, the waste will comprise organic and inorganic constituents andin most cases, it will be rich in organic materials. When the wastecomprises a combination of solids and liquids, the liquid portion shouldnormally not exceed about 30% by weight of the total.

As shown in FIG. 1, the waste is first introduced into a primarygasifier (12) which is a plasma furnace. This plasma furnace is normallya refractory lined, enclosed, graphite arc furnace, where the plasma isgenerated by one or several direct current electrodes forming anelectric arc, generally as shown in FIG. 2. The plasma is generated bythe electricity 16 flowing through graphite rods to a bath of liquidinorganic material, usually slag originating from the waste itself. Thisslag is maintained at a temperature of 1500° C. or more. Any metal (i.e.non oxidized inorganic material) present in the waste forms a distinctlayer below the slag layer. This metal layer is also maintained at hightemperature of 1500° C. or more. When starting the system, the slag canbe formed from a previous run or be a common inorganic material such assand or clay.

The organic material present in the waste reacts with primary air,oxygen and/or steam 14 that is added to the furnace using lances. Thisprocess is called gasification. The net result of the gasificationprocess is the production of a combustible gas called primary synthesisgas 18, containing CO, H₂, CO₂ and N₂ if the waste contains nitrogen orwhen the gasifier is fed with air, since air contains 21% O₂ and 79% N₂by volume. The primary synthesis gas also contains soot and some complexorganic molecules.

Gasification occurs as the results of a series of complex chemicalreactions that can be simplified as follows;

C+O₂->CO₂ (exothermic)C+H₂O->CO+H₂ (endothermic)C+CO₂->2 CO (endothermic)CO+H₂O->CO₂+H₂ (exothermic)

Some of the reactions are endothermic and some reactions are exothermic.The amount of oxygen, air and/or steam fed to the gasifier can beadjusted to balance the exothermic and endothermic reactions so as tominimize the amount of electric energy required in the furnace. Contraryto dissociation, gasification with metered amounts of oxygen, air and/orsteam requires minimal amounts of electrical energy to produce thesynthesis gas.

The slag in the primary gasifier 12 is covered with untreated andpartially treated waste, also called a cold top. This cold top servestwo purposes. First, since the slag is covered with the relatively coldpartially treated waste, the furnace roof and spool are not exposed tothe high radiative heat from the slag, reducing heat losses in thefurnace and increasing refractory life. Second, the cold top favours thecondensation of heavy metals onto the partially treated waste and theirsubsequent fusion into the slag. The slag 20 is periodically removedfrom the primary gasifier when required.

However, due to its relatively cold temperatures (700 to 800° C.), thecold top favours the production of complex organic molecules and soot(carbon) in the primary gasifier 12.

In order to trap the large soot particles, a dust separator 22 isinstalled at the gas outlet of the primary gasifier 12. Dust 24 that isremoved by the dust separator 22 is normally returned to the primarygasifier 12 for further processing.

The gas exits the dust separator 22, cleaned of large particulates(generally larger than 10 microns). However, it still contains fine sootparticulates and complex organic molecules. A secondary gasifier 26 isused to convert the soot and complex organic molecules to CO, H₂ andCO₂. The secondary gasifier 26 operates using electricity 28 in the formof a plasma torch at a higher temperature than the cold top, namelybetween 900 and 1300° C. and preferably around 1100° C. At this elevatedtemperature, the thermodynamic equilibrium between C, CO, CO₂, H₂ andH₂O, favours the formation of CO rather than the formation of C (orsoot). Also, at this high temperature, complex organic molecules areconverted to simpler molecules CO, CO₂ and H₂. Complex organic moleculessuch as products of incomplete combustion (PIC) are well knownpollutants and could be difficult to burn at lower temperatures. Thesecondary gasifier 26 ensures that they are converted to the inoffensiveCO and H₂ form.

The secondary gasifier 26 is equipped with a plasma-torch fired eductoras shown in FIG. 3. This eductor ensures that all the gas entering thesecondary gasifier 26 is exposed to the high heat and the high intensityradiation of the plasma flame. This ensures essentially completeconversion of all or substantially all the components of the synthesisgas entering the secondary gasifier 26 into simple gaseous molecules ofCO, CO₂, H₂ and H₂O.

Two measures are taken in order to ensure high energy efficiency of thesecondary gasifier 26. First, the plasma torch 28 provides theactivation energy for the conversion reactions, while small meteredamount of secondary oxygen, air and/or steam 30 is added, so that theenergy required to increase the gas temperature from 800 to 1100° C. isprovided mainly by the partial oxidation of the primary synthesis gas18. Second, the secondary gasifier 26 chamber is insulated with amaterial such as ceramic wool, in order to ensure minimal heat loss fromthe chamber.

The synthesis gas 32 exiting the secondary gasifier 26 is then cooled bycooling water using a water quench 34. In the water quench, the gas iscooled very rapidly, in a few milliseconds, from 1100° C. to below 100°C. This rapid cooling allows to freeze the thermodynamic equilibrium ofthe gas and, hence, to avoid the production of secondary pollutants suchas dioxins and furans. Dioxins and furans are mainly formed from therecombination of chlorine and carbonated compounds (such as CO and CO₂)in the gas. By cooling the gas quickly, this recombination does not havetime to occur. The gas is then subjected to gas cleaning 36 which may bea series of known unit operations that will remove remainingcontaminants from the gas such as: fine dust, heavy metals, acid gases(hydrogen chloride and hydrogen sulphide), etc.

The whole system is kept under a negative pressure by the use of aninduced draft fan 38. This ensures that no toxic fumes can exit thesystem and that the flammable H₂ and CO stay inside the system, limitingthe dangers of fires or explosions. The fan can be of turbine orpositive displacement type, depending on gas composition. Gascomposition will be a function of operating conditions and type of wastebeing processed.

The output of the system is clean combustible fuel gas, which can beused for different applications. First, it can be burned in a gas engineor gas turbine 40 for the production of electricity. In that case,cogeneration is also possible: the waste heat from the engine or turbinecan be used to produce steam and/or hot water. Depending on system sizeand waste type, the electricity produced by the engine or turbine may beenough to run the plasma arcs of the primary gasifier 12 and/or theplasma torch of the secondary gasifier 26. The gas can also be used as asource of heat for a boiler 42. In that case, the gas is burned in astandard burner, just as any other commercial gas such as natural gas orliquid petroleum gas (LPG). It can also be used for chemical synthesis44 as a reaction gas. In all these cases, since the fuel gas has beencleaned essentially of all contaminants, the emissions from the burningor processing of this gas will also be clean of any contaminants.

FIG. 2 illustrates the preferred embodiment of the primary gasifier 12.The solid and liquid wastes are introduced into the primary gasifier 12as a waste mixture through an isolation valve 46 and into one ormultiple feed chutes 48. Alternatively, liquid waste may be fed troughan injection nozzle 50 into partially treated waste 52 inside thefurnace. By feeding the liquid waste into relatively cold zones ofpartially treated waste 52, one ensures that the gasification reactionsof the liquid waste are progressive, rather than violent and sudden,which would occur if liquid waste were fed directly on top of the hotslag 20.

The waste is laid over a pool of slag 20 and molten metal 21. The slagand metal are maintained in a liquid state at a temperature of 1500° C.or more by the use of plasma arcs 54 and resistive heating (not shown).The plasma arcs 54 are generated by one or more graphite electrodes 56that carry DC electric current. Current typically flows from oneelectrode to the other when more than one electrode 56 is used, creatingan arc between one electrode tip 57 and the slag 20, then passingthrough the highly electrically conductive hot slag 20 and molten metal21 and creating a second arc from the slag 20 to the second electrodetip 57. The electrodes are typically submerged in waste 52, and theplasma arcs 54 are typically covered by waste 52. This favours thepassage of current inside the hot slag 20 and molten metal 21, ratherthan through gas, directly from one electrode to the other. The slag 20is covered with partially treated waste 52 also referred to as a coldtop. Fresh waste 51 is continuously or intermittently added as thegasification reactions in the furnace reduce the volume of waste 52present.

Waste 52 is heated by plasma arcs 54, which favour the conversion of theorganic components of the waste into CO and H₂. This process is referredto as the gasification reactions. Air, oxygen and/or steam are addedthrough a lance 58, in order to favour the gasification reactions in thehighest temperature zones of the primary gasifier 12.

The inorganic components of the waste melt and form two distinct layers:a bottom layer of the denser metal 21 and a top layer of the lighterslag 20. Once cooled, this slag 20 becomes a glassy rock, which can beused for construction or other purposes. The rock is non-leaching innature and allows to trap heavy metals and other contaminants into aglass matrix. Slag 20 and metal 21 can be extracted separately from thefurnace through two distinct tap holes 60 and 62.

In the primary gasifier 12, the organic molecules in the waste reactwith sub-stoichiometric amounts of oxygen, air and/or steam (i.e. lessthan the oxygen required for complete oxidation of the waste) to formthe primary synthesis gas 18. Steam used in the primary gasifier cancome from water already present in the waste or be added separately.

The primary synthesis gas 18 is normally composed of combustible CO, H₂and of non-combustible CO₂ and N₂. Since the slag is covered bypartially treated waste or cold top 52, the gases exit the primarygasifier at a relatively low temperature (800° C.). Because of therelatively low temperatures involved in cold top operation, the primarysynthesis gas 18 also contains soot and complex organic molecules (suchas ethylene, acetylene and aromatic compounds).

The advantage of cold top operation is higher energy efficiency for tworeasons: 1) the furnace spool 64 (top section) is kept at a lowtemperature and 2) the primary synthesis gas 18 exiting the furnace hasa lower temperature.

By keeping the spool 64 cold, the radiative heat losses to the roof aremuch reduced. The radiative heat losses are a function of temperature tothe 4^(th) power (q=εσ(T₁ ⁴−T_(Surr) ⁴)). In consequence, the effect ofcovering the slag by partially treated waste and reducing itstemperature from 1500° C. to 800° C. produces a reduction in radiativeheat loss of about 10 times.

Reducing the temperature of the primary synthesis gas 18 also reducesthe sensible heat of the gas exiting the furnace and, therefore, thesensible heat carried out of the furnace.

Another advantage of the cold top operation is to limit entrainment ofparticulates. Because the fresh waste 51 falls on a relatively coldsurface of the waste 52 being processed, the gasification reactions areless violent and happen in stages as the waste progresses down from coldtop temperature to reaction temperature of 1500° C. at the slag 20surface.

A still further advantage of cold top operation is to minimize thevolatilization of metals, volatilized metals at the high slagtemperature condense on the cold waste particles and have a betterchance of being trapped in the slag.

Due to the lower temperatures on the top of the reactor, some waste willexit the reactor unreacted or partially reacted. For example, some oilwaste will vaporize before being completely dissociated into CO and H₂.The thermodynamic equilibrium under the reducing conditions of thefurnace favour the production of carbon soot at the relatively lowtemperature at the outlet of the furnace (800° C.). A secondary gasifier26 working at around 1100° C. is used to convert any remaining complexorganics in the primary syngas to CO and H₂. It is shown in FIG. 3 ofthe drawings. The carbon soot is converted to CO by the addition ofoxygen, air and/or steam to the secondary gasifier. At 1100° C.,thermodynamic equilibrium, under reducing conditions, favours theproduction of CO, rather than soot (C).

The use of the secondary gasifier 26 also gives the option ofcontrolling the chemistry of the fuel gas or secondary synthesis gas 32produced by the system, without affecting the operation of the primarygasifier 12 (dust entrainment, electrode erosion, slag volatilisation).For example, adding steam into the secondary gasifier 26 will tend toincrease the amount of hydrogen present in the secondary synthesis gas32, while reducing the amount of carbon soot and carbon monoxide.

The secondary gasifier 26 includes a high temperature chamber 66,equipped with a gas mixer or eductor 68 at the chamber inlet. The insidewalls of the eductor 68 can have different construction:refractory-lined, water-cooled, or high heat metal alloy. The eductor isequipped with a plasma torch 70 at the inlet. The eductor 68 provides asuction effect on the primary synthesis gas and favours intimate contactof the soot particles and complex organic molecules with the plasmaflame in the eductor throat 69. The high temperature chamber isinsulated with insulation 67 in order to ensure minimal heat loss fromthe chamber.

The present invention is not limited to the specific embodimentsdescribed above, but may comprise various modifications obvious to thoseskilled in the art without departing from the invention and the scope ofthe following claims.

1. A two-stage plasma process for converting waste having organic andinorganic components into fuel gas, which comprises: (a) in the firststage, vitrifying or melting the inorganic components of the waste andpartially gasifying the organic components; and (b) in the second stage,completing the gasification of the organic components so that gas fromthe first stage of the process entering the secondary gasifier isexposed to a high temperature such as to transform essentially all sootpresent in the gas to CO and to convert essentially all complex organicmolecules to simpler molecules CO, CO₂ and H₂, wherein a dust separationand removal step is provided between the first and second stages of theprocess.
 2. A process according to claim 1, in which the fuel gasproduced in the second stage is quenched and cleaned to make it suitablefor use in a gas engine or turbine for production of electricity or in agas burner for production of steam or in chemical synthesis reactions.3. A process according to claim 1, in which the first stage is carriedout in a plasma arc furnace.
 4. A process according to claim 1, in whichthe second stage is carried out in a secondary gasifier using a plasmatorch with addition of metered amounts of oxygen, air and/or steam.
 5. Aprocess according to claim 3, in which the plasma arc furnace is arefractory lined, enclosed furnace provided with at least one directcurrent graphite electrode adapted to generate a plasma arc to a bath ofliquid inorganic material originating from the waste itself and locatedat the bottom of the furnace.
 6. A process according to claim 5, inwhich said liquid inorganic material comprises a slag layer which ismaintained at a temperature of at least 1500° C.
 7. A process accordingto claim 6, in which said liquid inorganic material further comprises ametal layer also maintained at a temperature of at least 1500° C. andlocated under the slag layer.
 8. A process according to claim 5, inwhich the waste is introduced into the furnace on top of the liquidinorganic material and the organic component in the waste reacts withair, oxygen and/or steam supplied to the furnace in a predeterminedamount adapted to achieve gasification of organic material in the wasteinto a primary synthesis gas containing CO, H₂, CO₂ and N₂ if the wastecontains nitrogen or if air is added to the furnace, and also containingsome soot and complex organic molecules.
 9. A process according to claim8, in which the organic material in the waste is so reacted as to form alayer of partially treated waste on top of the slag layer and freshwaste is introduced into the furnace on top of said partially treatedwaste layer which is maintained at a temperature of between 700 and 800°C. and constitutes a cold top for the fresh waste added to the furnace.10. A process according to claim 1, in which in the first stage, theorganic component in the waste reacts with air, oxygen and/or steamsupplied to the furnace to achieve gasification of organic material inthe waste into a primary synthesis gas containing CO, H₂, CO₂ and N₂ ifthe waste contains nitrogen or if air is added to the furnace, and alsocontaining some soot and complex organic molecules, and wherein theprimary synthesis gas is subjected to the dust separation and removalstep in which dust particles larger than a predetermined size areseparated and removed.
 11. A process according to claim 10, in which theremoved dust particles are recycled to the first stage.
 12. A processaccording to claim 4, in which the secondary gasifier is equipped with aplasma torch fired eductor for exposing the gas from the first stage ofthe process entering the secondary gasifier to a high temperature.
 13. Aprocess according to claim 12, in which the high temperature to whichgas from the first stage is exposed in the secondary gasifier is between900° C. and 1300° C.
 14. A process according to claim 13, in which thehigh temperature is achieved mainly by partial oxidation of the gas fromthe first stage by injection of predetermined amounts of air, oxygenand/or steam to the eductor, and the plasma torch provides only a smallfraction of the energy required for maintaining said high temperature.15. A process according to claim 12, in which the fuel gas exiting thesecondary gasifier is cooled down very rapidly to a temperature below100° C. so as to freeze the thermodynamic equilibrium of the fuel gasand avoid production of secondary pollutants.
 16. A process according toclaim 15, in which after cooling, the fuel gas is subjected to a finalcleaning operation to remove any remaining contaminants.
 17. A processaccording to claim 1, in which the process is carried out under anegative pressure to preclude exit of toxic fumes or of flammablematerials from any unit operations.
 18. A process according to claim 1,in which an oxygen starved environment in used in the process topreclude dioxin formation.
 19. Apparatus for converting waste havingorganic and inorganic components into fuel gas, which includes: (a) aprimary gasifier comprising a refractory lined, enclosed plasma arcfurnace provided with at least one graphite electrode; at least oneinlet for feeding waste into the furnace; means for feeding air, oxygenand/or steam in metered amounts into the furnace; and a gas take offport for primary synthesis gas produced in said primary gasifier; saidprimary gasifier being adapted to maintain layers of molten metal andmolten slag at the bottom of the furnace and on top of the molten slag alayer of partially treated waste on top of which fresh waste is fed; andsaid at least one graphite electrode being adapted to generate a plasmaarc to the molten slag present in the furnace during the operation; and(b) a secondary gasifier to which the primary synthesis gas is fed, saidsecondary gasifier being equipped with a plasma-torch fired eductoradapted to expose the primary synthesis gas entering from the primarygasifier to a high temperature such as to transform essentially any sootpresent in said primary gas into CO and to convert essentially anycomplex organic molecule to simpler molecules CO, CO₂ and H₂; means forsupplying metered amounts of air, oxygen and/or steam into the eductor;said eductor leading to an insulated chamber; and an outlet beingprovided in said chamber for the fuel gas resulting from the operation,wherein a dust separator is provided between the primary gasifier andthe secondary gasifier.
 20. Apparatus according to claim 19, in which inthe primary gasifier two graphite electrodes are used creating an arcbetween one electrode and the slag during the operation, and creating asecond arc from the slag to the second electrode.
 21. Apparatusaccording to claim 19, in which the eductor provided in the secondarygasifier is made of a high heat metal alloy or is refractory lined orwater cooled, and is equipped with the plasma torch at its inlet. 22.Apparatus according to claim 19, wherein dust particles removed by thedust separator provided between the primary gasifier and the secondarygasifier are recycled to said furnace of the primary gasifier. 23.Apparatus according to claim 19, further comprising a gas quenching andgas cleaning means following the secondary gasifier.
 24. Apparatusaccording to claim 19, further comprising an induced draft fan adaptedto operate the apparatus under a negative pressure.
 25. A processaccording to claim 11, in which the first stage is carried out in aplasma arc furnace.